D-psicose is a monosaccharide known as a rare sugar because it is rarely found in natural materials or is present in small amounts. D-psicose has very low calorie and a sweet taste similar to sugar, and thus is widely used as a functional sweetener.
D-psicose is an epimer of D-fructose, and has a degree of sweetness and taste very similar to D-fructose. Unlike D-fructose, D-psicose is scarcely metabolized in the body, and has almost zero-calories. D-psicose can be used as an efficient ingredient for diet foods since D-psicose has capabilities to inhibit activity of an enzyme involved in lipid synthesis and reduce abdominal obesity. Further, sugar alcohols such as xylitol and the like widely used as sugar substitutes may have side effects such as causing diarrhea when consumed in large amounts. On the contrary, D-psicose is known to have substantially no side effects.
For such a reason, D-psicose draws keen attention as a diet sweetener, and there is a growing need for a method of efficiently producing D-psicose in the food industry. As such, with an increasing need for development for D-psicose, various attempts have been made to produce D-psicose from D-fructose using existing biological methods. As enzymes capable of converting D-fructose into D-psicose, D-psicose 3-epimerase (DPE) derived from Agrobacterium tumefaciens and D-tagatose 3-epimerase derived from Pseudomonas cichorii or Rhodobacter sphaeroides are known. D-psicose 3-epimerase is known to have higher activity than D-tagatose 3-epimerase.
In production of D-psicose, more D-psicose is produced with increasing reaction temperature. However, in the case where a wild type D-psicose 3-epimerase is used in production of D-psicose, the enzyme is denatured under the reaction temperature of about 50° C. or more, which decreases enzyme activity, thereby causing a problem that the produced amount of D-psicose is decreased. Therefore, in order to efficiently produce D-psicose with high utility, there is an urgent need for a D-psicose 3-epimerase mutant with improved heat resistance.